1. Field of Invention
This invention relates to extrusion die structures and more particularly to extrusion dies for forming ceramic honeycomb structures.
2. Prior Art
Ceramic honeycomb structures made of cordierite, alumina or mullite show high mechanical strength and erosion resistance at high temperatutres. Since their manufacturing costs are low, their use with a coating of a catalyst material such as platinum, cobalt or manganese which convert toxic exhaust such as carbon monoxide and nitrogen oxides into carbon dioxide through oxidation and into nitrogen through reduction is increasing. Consequently, honeycomb type structures are being used for automobile exhaust systems and in the chemical industry as filters installed in catalytic cleaning towers or carriers for deodorant catalysts.
Honeycomb structures comprise a plurality of cores or cells colected in a desired configuration. The cross-section of the cells can be square, hexagonal, triangular, circular, or any desired geometric pattern. In FIG. 1 is given an example of a honeycomb article which is a ceramic honeycomb structure installed in an automobile muffler.
The ceramic honeycomb structure M has a cylindrical periphery, inside which a plurality of cell walls C having a thickness of between 0.3 mm and 1.0 mm are formed in a grid, and extend along the longitudinal axis of the structure. Accordingly, the ceramic honeycomb structure is a unitary body comprising a multiplicity of cell walls C.sub.1, C.sub.3, C.sub.5, etc., and C.sub.2, C.sub.4, C.sub.6, etc., arranged in rows and columns at an interval as short as the edge length of a single unit cell a. The cell walls enclose unit cells a which are arranged in close contact with each other and through which the exhaust gas flows. In addition, an outer casing or shell b is formed for protecting the thin-walled cells a so as to provide the honeycomb structure with sufficient mechanical strength.
Methods of making honeycomb structures and honeycomb extrusion dies are disclosed in U.S. Pat. No. 3,790,654 and No. 3,824,196, and are summarized below by referring to FIGS. 2 and 3. The extrusion die consists of a die body 1 having outlet and inlet faces 11 and 12. A plurality of feed passageways 2 communicate with the inlet face 12 and extend into the die body 1 to deliver extrudable material. A plurality of discharge slots 3 communicate with the outlet face 11 and extend into the die body, and are interconnected so as to form a gridwork (triangular as shown in FIG. 3). The feed passageways 2 are located at the intersections of the gridwork formed by the interconnecting discharge slots 3. The opening of 1.sub.2 of the discharge slots 3 is designed smaller than the opening 1.sub.1 of the feed passageways 2 so as to provide a suitable flow resistance to the extrudable material. About the gridwork formed by a plurality of feed passageways 2 and apart from said feed passageways, an circumferential orifice is provided so as to deliver a portion of the extrudable material for forming an outer casing b about the honeycomb matrix. Located about the periphery of the gridwork formed by the discharge slots 3 is a collar member 5 for providing flwo resistance to that portion of the extrudable material discharged from the orifice 21. The collar member 5 is threadably attached to the inner wall of the die body 1 and comprises a discharge opening 31 for forming the outer casing b. One core pin 6 in the die body 1 corresponds to one unit cell or core of the honeycomb article to be formed.
With the use of the extrusion die structure described above, ceramic articles are formed in the following steps as illustrated in FIG. 3. An extrudable batch material under pressure flows longitudinally through the feed passageways 2 as shown by the arrow A, and reaches that portion of the feed passageways 2 which communicates with the discharge slots 3. At this point, the spacing 1.sub.2 of the discharge slots narrows, thereby resisting the passage of the extrudable material to a great extent. Consequently, a portion of the batch material is caused to flow in lateral direction as indicated by arrows B.sub.1 and B.sub.2, and is finally discharged as a honeycomb structure having a coherent gridlike mass as indicated by arrow C.
The prior arts summarized above contain certain problems which are described hereinbelow. If a large number of fine cells are to be formed within a limited periphery of a honeycomb structure, a correspondingly large number of feed passageways with a larger opening that of the disclarge slot would have to be bored at an increasednumber of intersections of the an undesirable increase in manufacturing and labor costs. Furthermore, as the number of passageways bored in the die increases, the joint area 7 between core pin 6 and the die body 1 is reduced, thereby decreasing the mechanical strength of the attachment of the core pin 6 to the die body 1. Furthermore, the prior art die structure for forming a honeycomb structure with an outer case is extremely complex. Furthermore, it is extremely difficult to change the peripheral shape of the honeycomb structure unless separate die is prepared to fulfill the new design requirements. Also, with the large number of fine cells and the corresponding large number of feed passageways, it is difficult to maintain sufficient extrusion pressure in the die. Another problem is that since the honeycomb structure immediately after extrusion contains water an d is too soft to maintain its own structural integrity, the honeycomb structure frequently distorts under its own weight.
In light of the above, the present invention is primarily concerned with an improved means to eliminate the above-mentioned shortcomings.